Use of conductive-diamond electrochemical oxidation for wastewater treatment
Introduction
In the recent years, several works have been published concerning the application of conductive-diamond electrochemical oxidation (CDEO) for the treatment of aqueous wastes. Pollutants oxidized in the studies with synthetic wastes include cyanide [1], carboxylic acids [2], [3], alcohols, and aromatic and polyaromatic compounds [4], [5], [6], [7]. However, this list is even greater if the treatment of actual wastes is also considered. Thus, CDEO has been applied to the treatment of olive-oil mills [8], wooden manufacturing factories [9], [10], [11], surphactants [12] and even to the treatment of wastes consisting of merged effluents of different industries [13], [14]. In every case, a successful treatment has been obtained. Thus, it has been concluded that it is a robust and efficient technology which, in most cases, is able to achieve the complete mineralization of the organics contained in the wastes. The efficiency of the technology is very high, and it only seems to be limited by the transport of pollutants to the anodic surface.
Many approaches have been used to increase these efficiencies, such as the special design of cells [15], the combination of cathodically produced hydrogen peroxide with CDEO [16], or the use of ultrasounds [17]. All of them have shown good results, and have focused CDEO as a hot topic in industrial wastewater treatment and in the treatment of other wastewater such as those coming from soil remediation processes.
In addition, conductive-diamond electrodes show a great chemical and electrochemical stability, and an acceptable conductivity. The high overpotential for water electrolysis seems to be the more important property of conductive-diamond in its use in aqueous media. This electrochemical window is large enough to produce hydroxyl radicals with high efficiency, and this radical seems to be directly involved in the oxidation mechanisms that occur on diamond surfaces [18]. According to literature, direct oxidation models fit well the experimental data [19], [20], [21], [22], specially for non-chlorinated or nitrogenated substituted aromatics. However, it is known that in the electrochemical oxidation of wastewaters on conductive-diamond other oxidants are generated including persulphates [23], peroxophosphates [24], oxochlorinated anions [25] and hydrogen peroxide [26], depending on the waste composition and on the operation conditions. Thus, besides direct electro-oxidation on the surface and oxidation by means of hydroxyl radicals in a region close to the electrode surface, the oxidation mediated by other oxidants electrogenerated on the surface from the electrolyte salts should be taken into account, as it can complement the mechanisms of oxidation in this kind of electrochemical technology, and it contributes to increase the global oxidation efficiency.
Consequently, CDEO has shown better perspectives towards its application than other electrochemical oxidation technologies, and even that other advanced oxidation processes. In this context, the goal of this paper is to describe the more relevant figures of CDEO related to its use in industrial applications, and to point out the important role of the oxidation mechanisms that happen inside the electrochemical reactor and that shift CDEO as a very promising technology for the removal of organic pollutants in industrial wastes.
Section snippets
Wastewater characterization
In this work, both synthetic (propanol, phenol, hydroxybenzenes, chlorophenols, nitrophenols and dyes) and actual (pharmaceutical, olive-oil mills, chemical, petrochemical and door-manufacturing) wastewaters have been studied. In the case of electrochemical oxidation 5000 mg Na2SO4 dm−3 was used as supporting electrolyte.
Analytical procedure
The chemical oxygen demand (COD) was used to monitor the organic load of the wastes. It was determined using a HACH DR200 analyzer. Measurements of pH and conductivity were
Results and discussion
Fig. 1 shows the changes of the COD with the specific charge passed during the CDEO of four actual wastewaters. These effluents consist of aqueous wastes with a high concentration of organics, coming from the raw materials, intermediates and products of the different manufacturing plants (petrochemical, fine-chemical, door-manufacturing plants and olive-oil mills).
As it can be seen, and although the four wastes are very different in composition and organic load, the electrochemical process can
Conclusions
The main conclusions that can be drawn from this work are:
The electrochemical oxidation with conductive-diamond can be used to remove the organic content of a great variety of synthetic and actual wastewaters. Opposite to Fenton oxidation or ozonization, this technology does not lead to the formation of oxidation-refractory species and it allows diminishing the organic load of any effluent down to any required discharge limit.
The electrochemical oxidation of industrial wastes is strongly
Acknowledgment
The financial support of Spanish government through project CTM2007-60472/TECNO is gratefully acknowledged.
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